1. Field of the Invention
The present invention relates to an image pickup method and a control method for the same, more particularly to an image pickup apparatus equipped with an image pickup element having an electronic shutter function, and a shutter apparatus for shielding such an image pickup element and a control method for the same.
2. Related Background Art
Some conventional digital cameras achieve a viewfinder function that enables observation of an object to be photographed by sequentially displaying picked up images of the object on a monitor such as an LCD. In this type of digital cameras also, various shutter apparatuses can be used.
In the following, some conventional shutter apparatuses will be described.
Firstly, there is a shutter apparatus (a first shutter structure) used in silver-halide film cameras. This conventional shutter apparatus has two sets of diaphragm unit pivotally supported on a shutter substrate, in which multi-divided diaphragm units are rotatably held by two arms respectively to form a parallel link. One of the diaphragm units is composed of a plurality of front blades (which are also referred to as a front screen) for changing a shielded state of the shutter aperture into an opened state upon exposure from a shielded state. The other diaphragm unit is composed of a plurality of rear blades (which are also referred to as a rear screen) for changing the opened state of the shutter aperture into a shielded state upon exposure from an open state. The front blades are linked with a front drive member including a drive source such as a spring for moving the blades and an armature to be attracted and held by a control magnet. In the photographing standby state, the armature is caused by a charge lever to be in contact with a yoke attraction surface of the control magnet, so that the front blades are kept in a state for shielding the shutter aperture. The rear blades are also linked with a rear drive member having the same structure as that for the front blades. In the photographing standby state, the armature for the rear blades is caused by the charge lever to be in contact with a yoke attraction surface of the control magnet, so that the rear blades are kept in a state for leaving the shutter aperture open. After shutter release, the charge lever is retracted from its blade holding position, and the front blades and the rear blades are kept in their start positions by magnetic forces of the control magnets supplied with electric power. Then, power supply to the control magnet for the front blades is shut off, so that the front blades move to open the shutter aperture. After a predetermined time, power supply to the control magnet for the rear blades is shut off, so that the rear blades move to shield the shutter aperture. In this way, an image area is exposed with an exposure time. After running, the front blades and the rear blades are moved to the respective photographing standby positions in preparation for the next exposure.
Further, there is a focal plane shutter for a digital camera (a second shutter structure) as disclosed in Japanese Patent Application Laid-Open No. 2002-023220. In this structure, when power of the camera is turned on, a rear diaphragm that have shielded the shutter aperture is moved to a set position by a setting member (similar to the charge lever) to open the shutter aperture, so that the object can be observed by a monitor such as an LCD. Upon shutter release, the setting member operates to cause a front diaphragm to close the shutter aperture, and after completion of preparation for exposure, the setting member returns to the initial position before setting, prior to start of exposure operation, so that the front diaphragm and the rear diaphragm run for exposure.
Furthermore, there is a focal plane shutter for a digital camera (a third shutter structure) as disclosed in Japanese Patent Application Laid-Open No. 2001-215555. In this structure, in the photographing standby state in which power of the camera is on, a setting member brings a second drive member for a front diaphragm (the second drive member being adapted to receive a force for moving the front diaphragm from a drive spring) and a rear diaphragm drive member to their set states. By this operation, the rear diaphragm that has shielded the shutter aperture is moved to a set position, and the front diaphragm is left at its running completion position. Thus, the shutter aperture is in the open state to allow observation of the object by a monitor such as an LCD. Upon shutter release, the setting member returns to the initial position before setting in conjunction with the mirror-up operation. Through this operation, a first driving member for the front diaphragm that has been retained by a cam of the setting member is released to be free, so that the front diaphragm is operated by a weak setting spring to close the shutter aperture. After completion of preparation for exposure, the front diaphragm and the rear diaphragm run for exposure.
Japanese Patent Application Laid-Open No. H11-041523 discloses an image pickup apparatus that can perform motion picture photographing without suffering from smear and also perform still picture photographing with sufficient accuracy in shutter operations (a fourth shutter structure). In this structure, charge accumulation start scanning by an image pickup element is performed to function as an electronic front screen in conformity with the running speed of a mechanical shutter functioning as a rear screen. Then, running of the mechanical shutter as the rear screen is performed as closing running to enable an exposure operation in still picture photographing in the image pickup apparatus disclosed in this document.
In the above-described first shutter structure, when the object to be photographed is to be observed through the viewfinder function of the monitor such as an LCD, it is necessary that the front diaphragm is driven in such a way as to open the shutter aperture fully, since it is necessary that an object image is picked up by the image pickup element. In the conventional shutter apparatus, the front blade control magnet is adapted to attract the armature with power supply to a coil to hold the front blades at the running start position. When a start signal for starting the operation of causing the front blades to open the shutter aperture is received, power supply to the coil of the front blade control magnet is shut off, and contact of the armature and the yoke by attraction is released, so that the attraction surface of the magnet is left in an exposed (or uncovered) state. The attraction surface will continue to be in the exposed state until the photographer determines the composition and depresses the release button and the photographing operation is completed. There is a possibility that this state will continue for a very long time under certain circumstances. Therefore, the possibility that dusts attach on the attraction surface, which is considered to cause disadvantages such as that accuracy in the shutter time is deteriorated due to attraction failure or that the front blades cannot be held due to disability in holding the armature.
The rear blade control magnet also attracts the armature with power supply to a coil to hold the rear blades at the running start position. As described above, in the state in which the front blades have opened the shutter aperture fully in order to enable observation of the object to be photographed on the monitor such as an LCD, the charge lever has been retracted from its blades holding position. Accordingly, it is necessary that the coil of the rear blade control magnet is continuously supplied with power. This leads to an increase in the power consumption.
In addition, in the case where observation of the object to be photographed through the monitor such as an LCD continues for a long time, the temperature of the coil of the rear blade magnet to which power has been continuously supplied becomes high. If the photographer takes photographs in this state, accuracy in exposure time in high shutter speed exposures can be deteriorated, since operation characteristics of the rear blade control magnet are instable.
Further, when the photographer depresses the release button fully to start photographing, the front blades that have left the shutter aperture full open is firstly charged by the charge lever to move to the running start position prior to the above-described operations performed in silver-halide cameras after shutter release. After that, it is necessary to perform the process of supplying power to the coils of the front blade control magnet and the rear blade control magnet to attract the front and rear armatures to holds the front blades and the rear blades to the respective running start positions. Since the operation of charging the front blades is need to be performed after starting shutter release, actual exposure is delayed accordingly (to result in a large shutter time lag). Thus, photo opportunities may be missed and comfort in photographing may be impaired.
In the above-described second shutter structure, the operation of charging the front diaphragm needs to be performed after starting shutter release, and actual exposure is delayed accordingly (to result in a large shutter time lag). Thus, photo opportunities may be missed and comfort in photographing may be impaired.
In addition, existence of the mechanical front screen requires a space for accommodation thereof. This leads to disadvantages such as an increase in the camera size, complex structures with many parts and an increase in the cost.
In the third shutter structure, the front diaphragm charging operation after starting shutter release is not necessary to be performed. Accordingly, disadvantages the first and second shutter structures suffer such as delay in actual exposure, missing photo opportunities and impairment of comfort in photographing are eliminated. However, a larger space is needed for the structure for driving the front screen, since a mechanical front screen is used, the front diaphragm drive lever has a double structure, and the setting spring for quickly moving the front diaphragm to the start position has been added. Therefore, the structure is more complex and including a large number of parts, which leads to an increase in the cost.
In the above-described fourth shutter structure, there is no mechanical front diaphragm (front screen). Therefore, the disadvantage of the first shutter structure that attraction failure of the front diaphragm control magnet due to attachment of dusts on the attraction surface of the front diaphragm control magnet leads to deterioration in shutter time accuracy or failure in holding the front diaphragm due to disability in holding the armature is eliminated. Since the operation of charging the front diaphragm after starting shutter release is not necessary, disadvantages such as delay in actual exposure, missing photo opportunities and impairment of comfort in photographing are also eliminated.
However, in the above-described fourth shutter structure, how running characteristics of the shutter apparatus are to be detected has not been proposed specifically.
For example, the running status of the rear screen of a mechanical shutter may be detected by a pair of photo reflectors as disclosed in Japanese Patent Application Laid-Open No. 2001-235779. In this case, one of the photo reflectors is disposed at a position for detecting the edge of the rear shutter screen near the upper edge of the aperture (shutter aperture). The other photo reflector is disposed at a position for detecting the edge of the rear shutter screen near the lower edge of the aperture (shutter aperture). In this structure, detection of running of the rear shutter screen is completed at a point just before completion of exposure. Therefore, information obtained by the detection is not reflected in charge accumulation start scanning by the image pickup element, which functions as an electronic front screen, until the next shooting. If the next shooting is performed under the conditions same as those in the last shooting, still picture photographing can be performed with a sufficient degree of accuracy in shutter operation. However, there is no guarantee that the next shooting will be performed under the conditions same as those in the last shooting.
Conditions that may vary include the attitude (or orientation) of the image pickup apparatus (camera), temperature, humidity, electric power, number of operations, shooting interval etc. Therefore, there is the disadvantage that still picture photographing cannot be performed with a sufficient degree of accuracy in shutter operation.
The present invention has been made in view of the above-described problems and has as an object to shorten the shutter release time lag with a simple structure and enable shutter control with high accuracy in shutter time.